Method and device for polishing the surface of a gas turbine blade

ABSTRACT

The invention relates to a method and device for polishing the surface of a gas turbine blade. According to the invention, a metallic anticorrosive layer is polished by using a stream of dry ice whereby preventing the surface from becoming contaminated and obtaining cost advantages.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP02/12520, filed Nov. 8, 2002 and claims the benefit thereof.The International Application claims the benefits of German applicationNo. 01128915.4 EP filed Dec. 5, 2001, both of the applications areincorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The invention is in the field of gas turbine blades or vanes andinvolves the application or refurbishment of a protective layer orprotective layer system on gas turbine blades or vanes.

BACKGROUND OF THE INVENTION

Protective layer systems on gas turbine blades or vanes are shown, forexample, in U.S. Pat. No. 4,321,310, U.S. Pat. No. 4,676,994 or U.S.Pat. No. 5,238,752. Gas turbine blades and vanes are exposed to veryhigh temperatures. To enable them to have the required resistance tohigh temperatures, therefore, they are made from materials which areable to withstand high temperatures. Nickel- or cobalt-based superalloysare particularly suitable for this purpose. In general, a protectivelayer or protective layer system to prevent oxidation, corrosion and/orfor thermal insulation is also applied to the base body of a gas turbineblade or vane of this type. A metallic alloy of the type MCrAlY, where Mrepresents an element selected from the group consisting of Fe, Co andNi, Cr is chromium, Al is aluminum and Y represents yttrium or a rareearth element, is particularly suitable for this purpose. A ceramicthermal barrier coating is often applied to a corrosion-resistant layerof this type. This thermal barrier coating is highly thermally stableand is used to shield the metallic base body from direct contact withthe hot gas. A typical material for a ceramic thermal barrier coating isyttrium-stabilized zirconium dioxide applied, for example, byatmospheric plasma spraying (APS).

A corrosion-resistant layer of this type is often applied by means of aplasma spraying process. Consequently, it has a relatively highroughness, which is undesirable. It is therefore necessary to smooth themetallic protective layer. This requires either a time-consuminggrinding process, in which the gas turbine blade or vane is moved for aprolonged period of time in a trough containing abrasive bodies untilthe desired roughness has been set, or requires the use of asand-blasting process, in which solid particles, in particular corundumgrains, are deflected from a jet onto the surface and in the processsmooth the surface. The grinding process is not only time-consuming butalso entails the risk of prior uncontrolled removal of material atexposed edges. The known sand-blasting processes in particular have thedrawback that solid particles of the jet may become included in theprotective layer, thereby unacceptably reducing the surface quality.

U.S. Pat. No. 5,645,893 relates to a coated component having a base bodymade from a superalloy and having a bond coat and a thermal barriercoating. The bond coat includes a platinum aluminide and a thin oxidelayer adjoining it. The thin oxide layer contains aluminum oxide. Thisoxide layer is adjoined by the thermal barrier coating, which is appliedby means of the electron beam PVD process. Yttrium-stabilized zirconiumdioxide is applied to the bond coat. Before the bond coat is applied,the surface of the base body is cleaned by means of a coarsesand-blasting process. Alumina sand is used for the material-removingprocessing of the base body.

A further measure aimed at making a gas turbine blade or vane suitablefor use at very high temperatures is cooling by means of cooling air.This air is passed into an internal cavity in the gas turbine blade orvane, from where it takes up heat. This cooling air is generally atleast partially guided out of cool-air bores onto the surface of the gasturbine blade or vane, where it forms a protective film. The cooling-airbores are electrochemically eroded or drilled by means of a laser beamin the region close to the surface, as demonstrated in U.S. Pat. No.5,609,779. Drilling by means of a laser beam offers time and costbenefits but often leads to the formation of burrs and contaminationsformed from fused-on or remelted particles on account of the action ofheat. These burrs have to be removed if only on account of the need tomaintain an accurate geometry of the film-cooling bores, which hashitherto required a complex, manually operated process.

U.S. Pat. No. 3,676,963 has described a process for removing undesirableregions of thermoplastic or elastic materials in particular in inner,relatively inaccessible regions by means of an ice jet. A correspondingapplication by means of dry ice, i.e. solid CO₂, is disclosed by U.S.Pat. No. 3,702,519. The undesired regions are removed by supercooling,resulting in embrittlement of the plastic regions, by cold CO₂particles, and these regions are then removed by further particles.

DE-A 205 87 66 shows a process for cleaning metallic, radioactivelycontaminated surfaces by means of an ice jet. The use of dry ice is alsoproposed for readily soluble deposits on the surface.

U.S. Pat. No. 4,038,786 and the corresponding DE-A 254 30 19 disclose adevice which can be used to generate a dry ice jet with a favorableparticle size and shape and without aggregation of the particles.

DE-C 196 36 305 shows a process for eliminating coatings and coveringsfrom a sensitive substrate. These coverings include soot, moss,pollutant deposits and high-viscosity non-impact-resistant coatings onsubstrates such as wood, plastic foams or sandstone.

Gentle removal of the coverings or coatings from the sensitivesubstrates is possible by means of a dry ice jet.

Comparable applications for dry ice blasting, for example for removal ofsilicone seals or paints from, for example, plastic moldings or othershape-critical base bodies are described in the following articles:

“Trockeneis-Strahlreinigen” [Dry ice blast cleaning], A. Buinger,Kunststoffe 86 (1996) 1, p.58; “CO2 blast cleaning”, Ken Lay, RubberTechnology International '96, pp. 268-270; “Reinigen mitTrockeneisstrahlen in der Austauschmotorenfertigung” [Cleaning by dryice blasting in reconditioned engine manufacturing], Eckart Uhlmann,Bernhard Axmann, Felix Elbing, VDI-Z 140 (1998) 9, pp.70-72; “Dry-iceblasting for cleaning: process, optimization and application”, G. Spur,E. Uhlmann, F. Elbing, Wear 233-235 (1999) pp. 402-411;“Stoβkraftmessung beim Strahlen mit CO₂-Pellets” [Impact forcemeasurement during blasting with CO₂ pellets], Eckart Uhlmann, BernhardAxmann, Felix Elbing, ZWF 93 (1998) 6 pp.240-243.

SUMMARY OF THE INVENTION

The invention provides a process for removing ceramic material from thesurface of a gas turbine blade or vane in accordance with patent claim1, the gas turbine blade or vane having a metallic high-temperaturecorrosion-resistant layer, to which a ceramic thermal barrier coatinghas been applied. In the process, a dry ice jet of dry ice particles ispassed over the surface, so that material is removed from the ceramicthermal barrier coating by the action of the impinging dry iceparticles.

The invention is based on the surprising discovery that the dry iceblasting is suitable for removing a ceramic thermal barrier coating.

Previous applications of dry ice blasting have been based to aconsiderable extent on a thermal-mechanical action on a relatively softcoating. A covering or coating of this type is removed in pieces byflaking off as a result of cold embrittlement and subsequent kineticaction. By contrast, a ceramic thermal barrier coating consists of ahard, robust material. Furthermore, a ceramic thermal barrier coating isspecifically designed to withstand fluctuating temperatures and thermalstresses. For this purpose, it is customary to build up a columnarstructure which allows thermal transverse stresses to be compensatedfor. The ceramic thermal barrier coating should therefore in actual factbe insensitive in particular to attempts to remove it bythermo-mechanical means.

Removal by means of dry ice avoids contamination by foreign substances.Moreover, the metallic base body of the gas turbine blade or vane is notadversely affected, and moreover no material whatsoever is removed fromthe high-temperature corrosion-resistant layer.

Therefore, the surface of the gas turbine blade or vane can be smoothedor deburred easily and with a high quality, or may also have the entireceramic thermal barrier coating removed from it.

Smoothing by means of a dry ice jet on the one hand provides the timeand cost benefit of a sand-blasting process over, for example, agrinding process. On the other hand, however, the main drawback of aconventional sand-blasting process, namely contamination of the blade orvane surface with particles of the sand jet, is avoided. The dry iceparticles sublime immediately and without leaving residues on contact,with the result that there is no accumulation or chemical reaction ofany type. Furthermore, no blasting residues remain to be disposed of.

The ceramic thermal barrier coating preferably includes zirconiumdioxide, and even more preferably it is formed entirely fromyttrium-stabilized zirconium dioxide.

The process according to the invention may in particular also beemployed for refurbishment of a layer system of a gas turbine blade orvane. For this purpose, the entire old ceramic thermal barrier coatingis removed.

It is preferable for the smoothing to set a predetermined maximumroughness of the surface. The maximum roughness of a roughness averageis preferably less than 30 μm, more preferably less than 15 μm.

As has been stated above, cooling-air bores often open out at thesurface. For production reasons, burrs often remain in the region wherethese bores open out, these burrs in particular comprising remeltedmaterial if the bores have been drilled by means of a laser beam. It ispreferable for these burrs to be removed by means of the dry ice jet.

It is preferable for the smoothing to be carried out in fully automatedfashion. A multiaxis holder for the gas turbine blade or vane ormultiaxis guidance of the dry ice jet makes it possible to reach anyregion of the surface which is to be smoothed.

It is preferable for the dry ice jet to leave a nozzle at a pressure offrom 10 bar to 30 bar.

The embodiments described under sections A) to F) may also be combinedwith one another.

The invention also provides an apparatus for removing ceramic materialof a ceramic thermal barrier coating on the surface of a gas turbineblade or vane, having a holder for the gas turbine blade or vane andhaving a nozzle for emitting a dry ice jet, and having a multi-axismanipulation device for moving the dry ice jet relative to the gasturbine blade or vane in such a manner that completely automated removalof the ceramic material of the gas turbine blade or vane becomespossible.

It is preferable for the dry ice jet also to be used to clean the gasturbine blade or vane. Cleaning of this type is to be carried out inparticular prior to coating of a gas turbine blade or vane. Any impuritymay have an adverse effect on the bonding of the coating to be applied.With conventional cleaning processes, there is a risk of foreignmaterial being included in the surface to be cleaned as a result of thecleaning agents used, for example sand in the case of sand-blasting.This risk is avoided by the use of a dry ice jet, since the dry icesublimes without leaving any residues.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below with reference to anexemplary embodiment, in which, in some cases diagrammatically and notto scale:

FIG. 1 shows an apparatus for removing ceramic material from a gasturbine blade or vane,

FIG. 2 shows a longitudinal section through a surface region of a gasturbine blade or vane having a protective layer system, and

FIG. 3 shows a process for removing ceramic material from a gas turbineblade or vane.

DETAILED DESCRIPTION OF INVENTION

Identical reference numerals have the same meaning throughout thefigures.

FIG. 1 shows an apparatus 15 for removing ceramic material from thesurface of a gas turbine blade or vane 20. To produce a compressed-airflow, a screw compressor 1, a compensation vessel 2, an adsorption dryer3, a cooler 4 and a measurement system 5 are connected in series. Air ishighly compressed, in particular to a pressure of from 3 to 12 bar, inthe screw compressor 1. The compensation vessel 2 is used to stabilize aconstant mass flow. In the adsorption dryer 3, the air is dried, andthen cooled in the cooler 4. A measurement system 5 is used to recordthe compressed-air parameters.

The compressed-air stream is then fed to a pellet supply device 12, inwhich dry ice pellets 6 are stored. The dry ice pellets 6 are fed to thecompressed-air stream by means of a screw conveyor 7 via a star feeder 8and are fed with the compressed-air stream to a Laval nozzle 10 whichcan be moved by means of a robot 9. They emerge from the nozzle as a dryice jet 14 at virtually the speed of sound and strike a gas turbineblade or vane 20. The gas turbine blade or vane 20 is mounted in amulti-axis holder 22, so that the dry ice jet 14 can be guided onto anypoint on the surface of the gas turbine blade or vane 20.

FIG. 2 shows a longitudinal section through a surface region of a gasturbine blade or vane 20. An MCrAlY corrosion-resistant layer 32 hasbeen applied to a base body 30 made from a nickel- or cobalt-basesuperalloy. A thin bond coat 34 has been formed on thecorrosion-resistant layer 32. The bond coat 34 improves the bonding of aceramic thermal barrier coating 36 of yttrium-stabilized zirconiumdioxide applied to the corrosion-resistant layer 32.

A cooling passage 38 leads from an internal cavity (not shown) in thegas turbine blade or vane 20 to the surface 24 of the gas turbine bladeor vane 20. A trapezoidal opening region 40 is formed in the vicinity ofthe surface. The opening region 40 is drilled by means of a laser beam,which leads to partial melting of material. As a result, a burr 42 isformed and has to be removed. This is done by means of the dry ice jet14.

In the case of refurbishment of a gas turbine blade or vane 20 which hasalready undergone prolonged use, the ceramic thermal barrier coating 36is replaced. For this purpose, the ceramic thermal barrier coating 36 orresidues thereof is completely removed by means of the dry ice jet 14.

However, depending on the setting of the dry ice jet hardness, partialremoval is also possible, so that the surface 24 is smoothed to apredetermined roughness by the dry ice jet 14.

1-10. (canceled)
 11. A process for removing ceramic material from thesurface of a gas turbine component, comprising: compressing air; dryingthe compressed air; cooling the compressed air; feeding the dried cooledcompressed air to a dry ice supply device; feeding dry ice to the dryice supply device; releasing a mixture of the compressed air and the dryice towards the surface to remove from the ceramic material from thesurface by the impinging dry ice particles.
 12. The process as claimedin claim 11, wherein the dry ice particles are fed to the compressed airstream by a star feeder.
 13. The process as claimed in claim 11, whereinthe air is compressed to a pressure level of 3 to 12 bar.
 14. Theprocess as claimed in claim 11, wherein a compensation vessel is used tostabilize a constant mass flow.
 15. The process as claimed in claim 11,wherein the ceramic material comprises zirconium dioxide.
 16. Theprocess as claimed in claim 11, wherein the surface is smoothed by theremoval of the ceramic material.
 17. The process as claimed in claim 16,wherein a predetermined maximum roughness of the surface is set by theremoval of the ceramic material.
 18. The process as claimed in claim 11,wherein a cooling passage that extends through the surface having aproduction-related burr formed at least in part in an opening region ofthe cooling passage is removed by the dry ice particles.
 19. The processas claimed in claim 18, wherein the cooling passage is formed by a laserbeam.
 20. The process as claimed in claim 17, wherein a multi-axismanipulation device for moving the dry ice jet relative to the gasturbine blade or vane is used to remove the ceramic material and achievethe predetermined maximum roughness.
 21. The process as claimed in claim11, wherein the dry ice jet exits a nozzle at a pressure of from 10 barto 30 bar.
 22. The process as claimed in claim 11, wherein the ceramicmaterial is completely removed by the dry ice particles.
 23. The processas claimed in claim 11, wherein the dry ice is provided in pellet form.24. The process as claimed in claim 11, wherein the mixture ofcompressed air and dry ice pellets is released towards the surface atnear the speed of sound.
 25. The process as claimed in claim 11, whereinthe mixture of compressed air and dry ice is passed over the surface aplurality of times.
 26. An apparatus for removing ceramic material fromthe surface of a gas turbine component, comprising: a gas turbinecomponent holder; a compressor for compressing air; a dryer for dryingthe compressed air; a cooler for cooling the compressed air; a dry icesupply device to receive the dried cooled compressed air; a dry iceemitter for emitting a stream of dry ice within the compressed air; anda multi-axis manipulation device for moving the dry ice jet relative tothe gas turbine component.
 27. The apparatus as claimed in claim 26,wherein the gas turbine component is a blade or vane.
 28. The apparatusas claimed in claim 26, wherein the air is compressed to a pressurelevel from 3 to 12 bar.
 29. The apparatus as claimed in claim 26,wherein the surface is smoothed by the removal of the ceramic material.30. The apparatus as claimed in claim 29, wherein a predeterminedmaximum roughness of the surface is set by the removal of the ceramicmaterial.
 31. The apparatus as claimed in claim 26, wherein a coolingpassage that extends through the surface having a production-relatedburr is formed at least in part in an opening region of the coolingpassage is removed by the dry ice jet.
 32. The apparatus as claimed inclaim 31, wherein the cooling passage is formed by a laser beam.
 33. Theapparatus as claimed in claim 30, wherein the multi-axis manipulationdevice removes the ceramic material from the surface and achieves thepredetermined maximum roughness.
 34. The apparatus as claimed in claim26, wherein the dry ice jet exits the supply device with an initialpressure of from 10 bar to 30 bar.
 35. The apparatus as claimed in claim26, wherein the ceramic material is completely removed by the dry icejet.
 36. The apparatus as claimed in claim 26, wherein the dry ice isprovided in pellet supply form.
 37. The apparatus as claimed in claim26, wherein the mixture of compressed air and dry ice pellets isreleased towards the surface at near the speed of sound.
 38. Theapparatus as claimed in claim 26, wherein the mixture of compressed airand dry ice pellets is passed over the surface a plurality of times.